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SBFAQ Part 3: Color Appearance
Marc Green
3.1 What is the relationship between wavelength and color appearance?
3.2 What other factors affect surface color appearance?
3.3 How does background affect color appearance?
3.4 How does chromatic adaptation affect color appearance?
3.5 How does size affect color appearance?
3.6 How does color constancy affect color appearance?
3.7 What factors affect brightness?
3.8 How does brightness affect color appearance?
3.9 How does saturation affect color appearance?
3.10 How many colors is enough?
3.11 Does everybody experience the same colors?
3.12 Do people in all cultures see the same colors?
3.1 What is the relationship between wavelength and color appearance?
It is a common fallacy to believe that wavelength directly determines color appearance. It is true that wavelength is closely tied to apparent color in aperture conditions. When it comes to surface color and normal viewing, however, wavelength sometimes plays an astonishingly small role in determining color appearance.
The relationship between wavelength and actual hue is roughly:
620-730: red
590-610 nm: orange
550-580 nm: yellow
490-540 nm: green
450-480 nm: blue
380-440 nm: violet
Colors can depart from the wavelengths dramatically in surface color. In rough terms, the visual system does not judge absolute wavelength or brightness. Instead, it compares the wavelengths and brightnesses coming from different parts of the scene. It is the relative rather than the absolute spectral composition that usually determines perceived color and brightness. Under aperture conditions, there is no comparison with other parts of the scene, so wavelength is the dominant factor.
3.2 What other factors affect surface color appearance?
- Background (simultaneous color contrast )
- Chromatic adaptation (successive color contrast)
- Color constancy
- Brightness (Bezold-Brucke effect)
- Size
- Saturation (Abney effect)
3.3 How does background affect color appearance?
An object's background can strongly influence an object's perceived hue, saturation and/or brightness. The background can do the following:
- induce its complementary hue into the object: if the background is green, the object will appear redder; if the background is blue, the object will appear yellower, etc. The effect is strongest when the background is much more saturated and brighter than the object. Backgrounds have strongest effect on yellow and weakest on blue. This is called "simultaneous color contrast."
- reduce apparent saturation of similar hue: a very red background will induce green into an object. If the object itself is pink, the induced green cancels some of its redness. The general rule is that a highly saturated background will desaturate objects of the same hue and enhance saturation of objects with the complementary hue.
- induce or reduce brightness: a dark background makes an object look brighter and a bright background makes an object look darker. This is "simultaneous brightness contrast."
The picture demonstrates the contrast effect. Induced hue: the yellow ring looks redder on the green background.
Induced saturation: the pink ring looks redder on the green background.
Induced brightness: the gray ring looks brighter on the black background.
Hue and saturation effects on a computer screen are relatively weak because it is impossible to create highly saturated backgrounds. Green, in particular, is very desaturated.
- produce assimilation effects: this is the opposite of simultaneous contrast. Instead of producing contrast, the background seemingly spreads to the object.
In the example here, the white bars spread to make the blue look lighter and the black bars spread to make the same blue appear darker. This is logically enough called "assimilation" or the "spreading effect."
3.4 How does chromatic adaptation affect color appearance?
Chromatic adaptation occurs when the viewer has prolonged exposure to light of a particular color. The effect is sometimes called successive brightness contrast because the effects are similar to those of simultaneous color contrast:
- the complementary color is induced: For example, viewing a red field would make a subsequently viewed yellow or white object appear greenish. This is just a negative afterimage.
- reduce apparent saturation: For example, adaptation to a red field would then make a pink object appear whiter.
- induce or reduce brightness: For example, viewing a bright field would make a subsequently viewed object appear dimmer ("rapid light adaptation").
3.5 How does size affect color appearance?
As objects shrink, their colors become less distinct. Dark colors, such as blue, converge on black while bright desaturated colors, such as yellow, become whiter.
3.6 How does color constancy affect color appearance?
Objects generally retain their color when viewed in different light. This "color constancy" phenomenon would not be expected if wavelength determined color appearance. To see this, note that the wavelengths reaching the eye from an object depend on two factors:
- Wavelengths of light (spectral composition) falling on the object.
- Wavelengths which are reflected to the eye by the object
Different sources emit light with different spectral composition. The three most common light sources are the sun, tungsten filament bulbs and fluorescent bulbs.
Sunlight has about all wavelengths in equal amount (actually it's a little low in the long wavelength range). Tungsten filament bulbs are balanced heavily toward long wavelengths (red), while fluorescent light is heavy in the short wavelengths (blue). When looking at the same object in each of these three environments, spectral composition of light hitting the object and then reflected to the eye differs. We seldom perceive a color difference, reinforcing our intuition that color is an invariant property of an object and not something manufactured in our heads. Somehow, the visual system is able to discount differences in spectral composition of light sources. (Although film does notice the difference in illumination sources. Remember when you had to select between daylight and indoor color film? The indoor film compensated for the long wavelength balance of tungsten lights.) The effect is just the opposite of color contrast where two identical lights can appear different; here, two lights with very different spectral composition appear identical.
The common (but possibly incorrect) explanation is that the visual system selectively adapts to wavelength. There is a visual process called light adaptation, where exposure to light lowers sensitivity. For example, when you come out of a movie house into daylight, the world seems very bright. After a few moments, you adapt (lose sensitivity) and the world seems dimmer. This effect is wavelength specific. If exposed to long wavelength light, the viewer loses more sensitivity to long wavelength than to short wavelength. The "blue" cones don't care about the extra long wavelength and adapt less. The "white" of a TV set is somewhat blue. You don't notice this when watching TV at home because of chromatic adaptation, but the light coming from a neighbor's TV through his window has a distinct blue color.
This wavelength selective adaptation explains color constancy. Tungsten sources produce more long wavelength light. We therefore adapt more to long wavelengths and their effect decreases. Similarly, we adapt to short wavelengths in the fluorescent light. As a result, it is as if the object were illuminated by light with roughly the same flat spectral composition.
There are limits to chromatic adaptation. First, highly saturated light can change an object's appearance. Suppose an object appears red when it is viewed in white light. The object would still appear red in red light because its pigment reflects long wavelengths. However, if the object were illuminated with blue light, it would appear dark gray or black because it absorbs short wavelengths and does not reflect them back to the eye. Color constancy can compensate only for moderate variations in white light.
Second, Kruithof found that people's perception of color normality depends on overall light levels. At lower intensities, objects appear normal when ambient light sources are warm colors. Cool colors, on the other hand, create a harsh, flat and unnatural look. At high intensity, objects appear correct in somewhat ambient light.
3.7 What factors affect brightness?
A color's brightness depends on many factors:
- luminance: All things being equal, a light having more luminance or energy will appear brighter.
- spectral location. When luminance is equal, colors from the middle of the spectrum, green and yellow, appear brighter than red and blue.
- background: Objects appear brighter on a dark background and darker on a bright background.
- viewer adaptation level: A light looks brighter when the viewer has been adapted to a lower luminance level. For example, going from a dimly lit room to the outside makes the world appear very bright. Going back inside makes the room appear even darker.
- duration: Brightness increases with duration up to a limit of about 1/10 second and then declines a bit. It may seem counterintuitive, but a short flash of the right duration will seem brighter than a steady light (Broca-Sulzer Effect).
- size: Brightness increases with object size up to a limit and then becomes steady.
3.8 How does brightness affect color appearance?
Perceived hues turn yellower and bluer (Bezold-Brucke effect) at high brightness.
However, brightness change does not affect color matches. That is, if you matched a 580 nm. yellow by mixing red and green, then they would continue to look identical even if brightness increased (Grassman's third law.)
3.9 How does saturation affect color appearance?
Except for yellow and some blues, adding or subtracting white causes a shift in perceived hue (Abney effect). The direction of change varies with location in color space and is too complicated to summarize, but designers should be aware that changing saturation also causes alterations in the perceived hue.
3.10 How many colors is enough?
The precise answer depends on the type of images and the designer's goal. Generally, people asking this question mean: How many colors do I need to create smooth gradations and to avoid sudden color discontinuities? The higher the number of bits per pixel, the smoother and more naturally the image rendering in some images. If color depth is 8 bits, then there is a maximum of 256 different values which can be assigned to a pixel. This is adequate for good rendering of icons, line drawings, clip art, graphs and charts, and images where there is little smooth gradation of color.
Eight bits is also often adequate for natural images where high fidelity between the original and computer image is not critical or if the original image had relatively few colors. Look at the color histogram of the image - a plot of the number of pixels having each color. If the majority of pixels fall into a relatively small number of bins, then 8 bit color is probably sufficient, even for a natural image. Otherwise, a more faithful rendition of an image may require 16 or 24 bit color in order to create smooth color gradations. However, even 24 bits does not insure perfectly smooth gradations in gray level.
What is considered acceptable is often an aesthetic judgment with no objective answer.
The number of colors also depends on context. Any of the factors which impair color discrimination, low brightness, low saturation, short duration, etc., reduce the required number of colors. In addition, people are poorer at discriminating some colors than others. If the scene contains gradations of yellow or blue-green hues, highly discriminable color ranges, rendering will require more bits because viewers will readily perceive color discontinuities. Conversely, the same image with violet, purple and red gradations would require fewer bits because color discrimination is poorer.
Another common meaning of the question is: How many colors do I need to prevent color substitution and dithering? This is more a technical than a perceptual question, and goes into the murky world of palettes, palette managers and safe colors. However, basic color considerations are important in choosing a good dithering method. The purpose of dithering is to blend differently colored dots into a uniform color patch, so the viewer should not see each dot as an individual.
The achromatic system has much finer acuity than does the chromatic system, so the viewer is less likely to see dots if there is low achromatic and high chromatic difference. Moreover, color discrimination is better for larger objects, so use random dither patterns that intermingle dots of different color.
3.11 Does everybody experience the same colors?
When you and I look at the same apple, do we see the same red? Such questions are unanswerable because perceptions are private experiences. Science is based on analyzing publicly observable events, so there is no scientific way to know whether peoples' color experiences are the same.
Of course, we both say the apple is "red," but this proves only that we have both learned the social convention of applying the word "red" to whatever it is that we are experiencing when we view the apple. It doesn't necessarily mean that we are having the same experience. Interestingly, some color deficient people learn social conventions so well that they are not even aware that they have a color defect. Dichromats typically use the same basic 11 color categories as trichromats.
3.12 Do people in all cultures see the same colors?
As noted above, you can't ask about private perceptual experiences of other people. However, you can ask whether people in other cultures group colors into the same categories that we do. Although different cultures have widely different words and color naming conventions, the best guess is that everyone organizes color the same way. Berlin and Kay, for example, found that people across cultures linguistically make the same color distinctions as those found in Western cultures. Moreover, other studies show that if given color samples and merely asked to organize them into piles based on "similarity," people in other cultures group the samples into the same red, green, blue, etc. piles that Westerners do.
However, the aesthetic or emotional connotation of color can change with culture. Western culture, for example, associates black with death, while the Japanese associate death with white.
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The picture demonstrates the contrast effect. Induced hue: the yellow ring looks redder on the green background. Induced saturation: the pink ring looks redder on the green background. Induced brightness: the gray ring looks brighter on the black background. Hue and saturation effects on a computer screen are relatively weak because it is impossible to create highly saturated backgrounds. Green, in particular, is very desaturated.
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Copyright © 2024 Marc Green, Ph. D.
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